This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning machine and method that construct text or images from individual colorant spots created on a printing medium, in a two-dimensional pixel array. Thermal-inkjet printers and processes are of greatest interest; however the invention is applicable in other types of units such as, for example only, piezodriven inkjet printers and hot-wax transfer printers.
(a) Mechanisms of color-intensity gradationxe2x80x94An incremental printer forms an image on a printing medium by placing colorant thereon in the form of tiny dots. An inkjet printer, for instance, ejects ink droplets that fly across a narrow gap to the print medium and so form the dots on the medium.
A principal way to reproduce color tones is by varying the areal density, i. e. number per unit area, of such dots on the medium. This variation ranges from none (white, if the print medium is white) to full density.
The phrase xe2x80x9cfull densityxe2x80x9d means that every location in the pixel array contains at least one dot. Making a color as intense as possible requires full density, to cover the printing medium with colorant.
Another way to produce various color tones is to vary the density of the colorant itself, i. e. the concentration of the colored substance that makes up each dot. This can be done for instance by applying plural quanta of colorant at a common pixel location.
Here the phrase xe2x80x9cquanta of colorantxe2x80x9d means the discrete units in which a printing system transmits colorant toward a printing medium. In inkjet and other liquid-ink systems, colorant quanta thus are drops of ink.
Variation in color tones can also be accomplished by storing colorants at various concentrations within the printing mechanism, and using those colorants in printing. This approach is relatively more costly and is not taken up further in this document.
(b) Texture and resolution vs. dynamic-range stabilityxe2x80x94The smaller the dots, the less visible are individual dots, thus producing an image that is less grainy and more realistic. These effects are desirable for ideal image quality at the low end of the dynamic range of color intensityxe2x80x94namely for very light colors such as highlight regions in photo-quality images, pastel washes and the like.
Certain factors, however, may cause the dots to be different in diameter than the nominal. Holding a nominal dot size is more difficult for smaller dots, and failure in this regard can introduce major problems, particularly at the opposite end of the dynamic rangexe2x80x94in other words, in densely packed regions where color should be very intense.
Among the sources of divergence from nominal dot size are use of different media types, printing under differing environmental conditions, and variations in parameters of colorant quanta as defined earlier. Inkdrops for example vary in weight, volume, viscosity, cohesion, adhesion (including electronegativity and chemical affinities), pH, temperature, physical integrity (some drops break into pieces having various relationships), shape or shapes, dimensions, speed, and directionxe2x80x94as well as molecular weight, shape and dimensionsxe2x80x94and all of these can affect resulting dot shape or size.
If dots are smaller than nominal, white spaces may appear in the areas where maximum color intensities should be, thus reducing such intensity through poor coverage. On the other hand, if dots are larger than nominal, the associated excess colorant leads to oversaturation of the printing medium with colorant or vehicle (particularly for colorant that is a liquid ink), which commonly creates undesirable image-quality defects.
Therefore dots cannot simply be made too small (or too large) so as to be on the safe side. It is important that dot size be controlled as well as practical or cost-effective.
Pursuing that objective, system designers often select the nominal dot size in such a way that variations about that nominal value minimize loss of image quality. Usually neighboring dots slightly overlap, to provide some allowance for error in case of slight misplacement or the chance of producing smaller dots than expected due to the variables that affect dot size.
In the end, dot-size selection is limited by the fact that positive variations produce oversaturation of colorant, again particularly for liquid ink. With that limitation, the native resolution of the system determines the dot size and in consequence the visibility of the individual dotsxe2x80x94permitting only relatively little control over texture quality in highlight regions, as noted above.
(c) Nonlinearityxe2x80x94The foregoing discussion relates to the difficulties of obtaining nominal dot size and therefore stable dynamic range. Even with correct dot size, however, in single-bit incremental printing several factors make linear response 211, 212 (FIG. 1a) of color intensity very difficult to obtain.
From the FIG. 1a graph, some of these factors can be appreciated based on geometrical relationships between a pixel grid 201 (FIG. 2a) and a correctly sized dot 202. For tutorial purposes, two simplifying assumptions have been adopted in the graphs.
The first is that the distribution of dots in some analyzed uniform-intensity region of the imagexe2x80x94i. e., for some moderately large portion of the pixel gridxe2x80x94at fifty-percent input intensity (0.5 on the abscissa) is a simple alternating-pixels checkerboard pattern. With such a dot distribution, if the dots are all correctly positioned there is no overlap of dots anywhere in the image region.
The second assumption is that, at all lower input intensities (zero through 0.5 on the abscissa), too, the dots are distributed with no overlap in the image region. As a result the perceptual response is simply the fraction of that image region which is covered with colorant.
The lower quadrant of the graph shows an idealized but nearly realizable response (i. e. perceived intensity) to an idealized progression of input-data intensities. The curve corresponds to placing drops of diameter d in a square pixel grid of pixel-size s (FIG. 2a), such that the diameter d just fits across the diagonal of the pixel.
Because the pixel square is exactly inscribed within the dot circle, d=sxc2x7xe2x80x22, and the dots do not overlap along diagonals. If there are i such drops within a grid region of n squares, then the fractional area covered by the drops is:             fractional      ⁢              xe2x80x83            ⁢      coverage        =                                        π            ⁢                          xe2x80x83                        ⁢                          r              2                                            s            2                          ·                  (                      i            n                    )                    =                                    π            4                    ·                                    (                              d                s                            )                        2                    ·                      (                          i              n                        )                          =                  k          ⁢                      (                          i              n                        )                                ,
k being a constant. Thus the printer response is linear. Inserting the value d2=2s, the coverage fraction becomes             π      4        ·          (      2      )        ·          (              i        n            )        =                    π        ⁢                  xe2x80x83                ⁢        i                    2        ⁢        n              .  
This applies from i=0 (zero input intensity) up to the point where i=n/2 (fifty-percent input intensity). There the expression reduces to xcfx80/4≈0.785, thus establishing the rectilinear portion 211 of the curve.
If there were no sources of nonlinearity for higher values, the curve would continue along the dashed, angled straight line 212 to xcfx80/2≈1.57. In that upper half of the input-intensity range, however, a progressively greater fraction of the space in the image region is occupied by overlapping chordal areas of the circular dots.
The dashed straight line 212 represents a fictitious case in which the colorant deposited in an overlapping area has just as much coloring power as if it were printed in two separate nonoverlapping spaces. In actuality the first-deposited colorant in those overlapping areas is muted by the overprinted additional colorant, so that its effect is substantially lessened.
On the other hand, if it had no effect at allxe2x80x94in other words, if the incremental effect of overprinted additional colorant were zeroxe2x80x94then the only impact of adding the second half of the input intensity would be to fill in the previously unprinted, pincushion-shaped blank spaces between dots. The area previously filled was 0.785 of a full 1.0 available area, so the response in this second fictitious case would move along the more-shallowly inclined straight line 213 to reach perceived intensity 1.0 at input intensity 1.0.
In reality the overprinted additional colorant has some positive effect, but as already stated the overall additive effect is muted so that the actual curve followed is between the two straight dashed lines 212, 213. Its slope tails off, very generally as suggested by the curved dashed line 214.
That dashed curve is not intended to represent actual performance quantitatively, but only to convey a general impression of the intensity response of such a system. In actuality the lower quadrant, too, of the response curve is idealized.
In practice, dithering and other characteristics of a system typically do not permit filling in dots in an absolutely systematic fashion leading to a checkerboard pattern at fifty percent. Therefore in a real case some overlapping occurs even in the first quadrant and typically leads to a rolling-off of the curve well before the fifty-percent point.
Furthermore, certain other influences exert influences on the response that are not at all purely geometricalxe2x80x94and unfortunately these greatly strengthen the tendency of the slope to tail off. In fact a pronounced xe2x80x9ckneexe2x80x9d shape develops in the response curve, leading to nearly a plateau near the full-input-intensity point.
One of these influences is dot-placement error (DPE). Such error increases the risk of having a knee, even with dot diameter equal to the pixel width or height (FIG. 2b). Where the relationship is as already discussed, and shown in FIG. 2a, the knee effect is pronounced.
The reason is that overlapping dot areas have less coloring power than the decoloring power of the relatively very large white spaces left unprinted when a dot is shifted out of position by, say, a half pixel width or more. The greater the DPE, the more noticeable the knee.
Finally, in liquid-ink systems there are strong colorant-to-printing-medium interactions that strongly affect the resulting diameter of the dots, depending on the dot density. Some media allow the dots to grow more when the medium is wet.
Drops first landing on the medium wet it, and drops landing thereafter grow more than they would if they were landing on dry medium. That dot-growth effect favors the creation of the knee, since the effective diameter of dots in high-density areas is larger than the diameter of dots measured in low-density areas exhibiting less interaction.
In addition to dot growth, other colorant-to-medium interactions include variation of the penetration depth of the colorants into the medium. This is a function of the moisture content of the medium, and consequently is influenced by the dot density.
The higher the dot density, the more wetting, the more penetration, the less colorant on the surface, and the less light absorbedxe2x80x94high liquid saturation leads to low chromatic saturation. These phenomena too generate strong knee effects.
As a result the actual response is only slightly above the dashed straight, but angled, line 214. Also the rolloff into the knee regionxe2x80x94over a transition region from the straighter lower-quadrant behaviorxe2x80x94is more gradual than might be suggested by the sharp angle graphed at the fifty-percent point.
These variations all add up to an unsatisfactory chromatic-saturation performance in which the upper half of the response is strongly collapsed or depressed, so that it is disproportionate to the lower half. Fully saturated input colors do not look less saturated, as printed, than half-saturated colorsxe2x80x94but they do not appear much more saturated, either.
Relatively subtle, developing colors, that should be trending toward a strongly saturated portion of an image, for example, may show great promise of things to come. The promise is there, but the punch is gone.
Another kind of disappointment is graphed by the straight dashed line 221 (FIG. 1b), representing performance related to the geometry between a pixel grid and a relatively smaller dot 204 (FIG. 2b) that just fills its pixel horizontally and vertically, rather than along a diagonal. Thus the drop diameter in this case is d=s. Here the same simplifying assumptions have been adopted as described abovexe2x80x94particularly use of a simple checkerboard pattern at fifty-percent fill, and systematic non-overlap of dots below that point.
(Although a common pixel size is drawn in all four views in FIG. 2xe2x80x94with different apparent dot sizes 202, 204, 206xe2x80x94the drawing is intended to show only relative sizes of the dots, as compared with the sizes of the associated pixels. Thus the dots 202, 203 in FIG. 2a are not necessarily large dots in an absolute sense; they and their unit pixel 201 could both be either very small or very large. Likewise those dots 202, 203 could be smaller than the dots 204-209 in the lower views, or all three could be the same absolute size.)
Now in FIG. 1b it is the entire graph, not only the lower-left quadrant, that shows an idealized but nearly realizable response. Because the pixel square is exactly circumscribed outside the dot circle, the dots if correctly positioned do not overlap at all, anywhere in the image region.
If there are i such drops within a grid region of n squares, then the fractional area covered by the drops is:             fractional      ⁢              xe2x80x83            ⁢      coverage        =                                        π            ⁢                          xe2x80x83                        ⁢                          r              2                                            s            2                          ·                  (                      i            n                    )                    =                                    π            4                    ·                                    (                              d                s                            )                        2                    ·                      (                          i              n                        )                          =                  k          ⁢                      (                          i              n                        )                                ,
as before, and the printer response again is linear. Inserting the value d=s, however, the coverage fraction for this case becomes             π      4        ·          (      1      )        ·          (              i        n            )        =                    π        ⁢                  xe2x80x83                ⁢        i                    4        ⁢        n              .  
This applies from i=0 (zero input intensity) all the way to the full-intensity point, i=n (one-hundred-percent input intensity). There the expression reduces to xcfx80/4≈0.785, the same value as found before but now at the 1.0 point on the abscissa rather than the 0.5 point.
The rectilinear idealized curve 221 spans the entire dynamic range of the printer. Here the problemxe2x80x94as seen based on geometry onlyxe2x80x94is not a knee but simply that the saturation is poor, and this plainly due to the large uncolored spaces in the corners 226 of all the pixels.
The poor saturation suggested in the graph is real. In actuality, however, even with a theoretical d=s situation, the DPE and colorant-to-medium effects explained above strongly favor the development of a knee in the curve, and again the more the DPE, the more noticeable the kneexe2x80x94though of course less pronounced than in the situation of FIG. 1b. Thus this case like the first is non-linear, and as a result also has poorer saturation than indicated by the graph.
Yet a third type of defeated expectation is symbolized by the straight dashed line 223 (FIG. 1c), representing performance for a dot shown in the solid line 206 or 208 (FIGS. 2c, 2d) that is smaller yet, in comparison with the pixel gridxe2x80x94so small that it fills only a fraction of its pixel. Such small dots are advantageous for printing on certain types of printing media.
The drop diameter in this case is d less than s, and purely as an example roughly d=xc2xes. The same simplifying assumptions as for FIG. 1b are adopted here to show the response as rectilinear across the graph.
In a system with such extremely small relative dot size, there is little or no knee effect. The intensity response, however, is far from satisfactory: it is very inadequately saturated throughout the input intensity range.
The reason for this is the even larger uncolored space of FIGS. 2c and 2d. Such a system may be capable of excellent highlights, and may exhibit very little grain if the absolute size of the dots is low, but most users would object that it prints images badly washed-out.
(d) Single-bit systemsxe2x80x94A system that is most economical minimizes the amount of data needed to encode the image at the time of printing. The minimum possible amount of data is one bit per colorant and per pixel, an advantageously modest data requirement.
The preceding subsection, however, discusses nonlinearity and deficient saturation in such systems. It demonstrates functional inadequacies which in general are a stiff price to pay for the associated data economyxe2x80x94and these are added to the problems of dynamic-range stability discussed earlier.
With this type of system, moreover, the only adjustmentxe2x80x94per type of printing mediumxe2x80x94possible heretofore is multiplication, by a fixed integral factor, of the number of dots per colorant and pixel location. This lack of flexibility is another major drawback.
For that adjustment, the limitations already discussed are applicable: dot size is fixed based on coverage and saturation for the colors of maximum intensity. Accordingly such an adjustment is very coarse, usually being limited to a selection between one dot or two, per colorant and pixel.
In purest principle perhaps a way to overcome these problems of linearity, saturation, dynamic range and adjustabilityxe2x80x94in a single-bit systemxe2x80x94might be to use extremely small dots and double up in more-intense pixels. Some such schemes may be seen in FIGS. 2c and 2d, from which it can be appreciated that even with these arrangements a significant unpainted space remains in each pixel.
What is more, specifying all the additional dots needed to implement this strategy gives away the sole advantage of the single-bit system, namely the data economy. For these reasons and others, the approach does not appear to be a practical option.
As can now be recognized, the classical single-bit has great appeal. Nevertheless the system leaves much to be desired.
(e) Plural-bit encoding systemsxe2x80x94Another, more sophisticated color-representation data management approach, known as color resolution enhancement technology (xe2x80x9cC-Retxe2x80x9d), uses several data bits per colorant and pixel location. In C-Ret more than two levels of colorant can be specified and placed at each pixel. For example, four combinations can be specified from two bits: 00, 01, 10 and 11.
Each bit combination can be associated with a different number of colorant quanta, and the combinations need not be a direct binary representation of the number of quanta. For example xe2x80x9c00xe2x80x9d may represent no colorant, xe2x80x9c01xe2x80x9d and xe2x80x9c10xe2x80x9d may represent one quantum and two quanta respectivelyxe2x80x94but xe2x80x9c11xe2x80x9d may be used to represent four quanta or even five. The code, however, establishes explicitly what quantities of quanta are to be provided; and the code as such is implemented directly.
Thus C-Ret controls the number of colorant quanta placed in each pixel. In this system the size of an individual single-quantum isolated dot can be much smaller than the effective dot size of composite dots in high-density areasxe2x80x94i. e., dots made from several individual colorant quanta.
Such a system provides very small individual dots, less visible individually and so contributory to excellent print quality in highlights. At the same time it provides desired coverage and color intensity in regions of vivid color. This method allows for an adjustable maximum colorant amount by remapping, i. e. redefining, the number of colorant quanta (per location and per associated colorant type) that is respectively associated with each bit-value combination.
C-Ret is thus capable of introducing into the system a nonlinearity that can be made opposite in effect to the undesired knee effects described above, and so can be made to compensate for such undesired effects. Of course several passes (or slower scan speed) are needed to place several colorant quanta in a common pixel location. Such operation, particularly including distribution of colorant application as among plural passes, is discussed in a later part of this xe2x80x9cBACKGROUNDxe2x80x9d section.
It is perhaps most natural to integrate C-Ret and other plural-bit systems into conventional printmasking regimes. The result is to parcel out the total number of colorant quanta into two or more passes in a conventional way.
(f) Bandwidthxe2x80x94In a single-bit system, as discussed above, the amount of data needed to encode the image at the time of printing is determined, by definition, as one bit per colorant and per pixel. This specification defines the system needs in terms of bandwidth for data processing, storage and transmission.
In the single-bit system, use of smaller dots heretofore requires more data to fully cover the printing medium when color of maximum intensity is required. This drawback is in addition to the problems of dynamic range introduced earlier.
The C-Ret system has substantially the same drawback. If two data bits are used, the number of data bits is simply doubled at the outsetxe2x80x94as compared with a conventional single-bit system operating with larger dots.
If C-Ret is used with a greater number of bits, for even better dynamic-range adjustment or further nonlinear effects, then the data usage is expanded even more severely. The same is true of all other plural-bit systems.
Thus for printing with small dots, all conventional single-bit systems and plural-bit systems alike have a major drawback. To encode the image at the time of printing, at any given resolution, requires more dataxe2x80x94and hence more bandwidth for data processing, storage and transmission.
(g) Depletionxe2x80x94It is known to selectively remove colorant quanta from portions of an image where excessive colorant would otherwise be deposited. Such intervention, known as depletion, is customarily employed as an essentially final adjustment after completion of all other image-processing steps including color corrections, gamut scaling or other gamut-matching efforts, dithering or error diffusionxe2x80x94but before printmasking and before machine-language control of the print engine.
Many patents have been issued on variations and refinements of the depletion function. The previously mentioned patent of Wade, for example, teaches a strategy of using inkdrop-volume measurements to fine-tune depletion. (Wade""s interest is exclusively in managing the overall colorant deposition of an entire printhead; he makes no suggestion of selective control for any particular group of pixels in an image.)
Wade indicates that it is currently believed preferable to control overall colorant deposition through depletion algorithm techniques, and also that servoing colorant density by direct control of drop volumexe2x80x94by varying printhead temperature, in particularxe2x80x94runs a risk of adversely complicating the energy management of the head.
Depletion-algorithm techniques, on the other hand, avoid such risks and can accomplish the same results of controlling inking volume. Usually used to avoid depositing excessive colorant, such procedures edit out colorant spots from the pixel-array pattern to be created on the printing mediumxe2x80x94but system designers try to implement this in an inconspicuous way that interferes as little as possible with the desired appearance of the image.
Depletion as universally practiced is thus a subtractive process, and not addressed to problems of color-intensity linearity. Castle and Lund, for example, in their above-noted patent describe using such a procedure to implement their determination of optimum print density in an inkjet printer. Their method includes analyzing nearby pixels in dense regions, to find candidate pixels for drop omission.
Their discussion makes clear that their concern is for gross liquid-overload problems as suchxe2x80x94xe2x80x9csmearing or blotting between pagesxe2x80x9d etc. They also mention in passing having xe2x80x9ccontemplated . . . an additive method that would apply additional drops of ink.xe2x80x9d Castle and Lund do not, however, suggest how such a method might be performed.
(h) Colorant additionxe2x80x94Colorant-additive procedures are known, but for entirely different purposes. The earlier-identified patent of Berge, for instance, teaches how to underprint a layer of chromatic colorant below a layer of black colorant. His goal is to avoid certain colorant-to-print-medium effects that cause generation of a color halo just inside the edge of a black field that directly abuts a chromatic-color field.
Berge explains that his method deters formation of the halo by chemically xe2x80x9cpreconditioningxe2x80x9d the surface of the printing medium. Berge""s presentation appears to suggest that the chromatic colorant is imbibed into the black colorant.
Perhaps his preconditioning exhausts an affinity of the medium (or of the black colorant) for the chromatic colorant. To carry out his scheme, Berge""s system analyzes the profile of solid-black fields, and further tests their proximity to solid-chromatic-color fields, to select pixels for underprinting with the chromatic colorant.
Berge""s patent does not suggest applying any such procedure to problems of colorant-density nonlinearity, or of deficient color intensities. His objective is very narrowly defined.
(i) Conclusionxe2x80x94Thus intensity shortfalls, and nonlinear colorant densities, have continued to impede achievement of uniformly excellent incremental printingxe2x80x94at high throughputxe2x80x94on all industrially important printing media. Important aspects of the technology used in the field of the invention accordingly remain amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits. Those facets are discussed in turn below.
In the process of implementing the present invention, a strong functional relationship to the depletion process has been discovered. The invention, however, has opposite objectives and opposite effect. Still, because of this kinship, which appears at the functional or operational level only, a coined nicknamexe2x80x94xe2x80x9cpropletionxe2x80x9dxe2x80x94has been applied to the invention.
In preferred embodiments of a first of its facets or aspects, the invention is a method of printing an image, by construction from individual marks formed in a pixel array on a printing medium, based on pixels of a corresponding input data array. This method includes the step of applying all data for a particular colorant, in the input data array, one time to control deposition of the particular colorant in forming the pixel array on the printing medium.
The method also includes the step of applying all data for the same particular colorant, in the data array, at least another time. This second application is to control deposition of an additional positive quantity of the same particular colorant in forming the pixel array on the printing medium. At least one of the applying steps includes the substep of selecting particular pixels of the data array to control deposition of the same particular colorant.
The foregoing may constitute a description or definition of the first facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention significantly mitigates the difficulties left unresolved in the art.
In particular, this novel method enables printing of a fuller dynamic range of colorant densityxe2x80x94without paying the data-bandwidth penalties previously supposed unavoidable. That is, the system can provide an extended density range with smaller data files to be stored, copied, and transmitted between different computers.
Although this aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits.
For example, it is preferred, particularly if the method is for use in a scanning printer, that the xe2x80x9cpixel-array forming one timexe2x80x9d include depositing colorant in one pass of the scanning printer; and the xe2x80x9cpixel-array forming another timexe2x80x9d include depositing colorant in another, different pass of the scanning printer.
It is alternatively preferred, particularly for a scanning printer having a scanning printhead that makes plural passes across the printing medium, that the pixel-array forming one time and the pixel-array forming another time both direct colorant-deposition control data to a printmasking stage; and that the printmasking stage allocate colorant deposition in particular pixels among the plural passes. In this case, it is further preferred that the selecting substep include defining a maximum density for the pixel array of the printing medium.
Pursuing this same preference still further, it is preferable that the selecting substep include identifying locations of the particular pixels to receive said maximum density. In this case it is additionally preferred that the identifying include analyzing pixels of the data array to find locally dense areas. Further yet, it is desirable that this analyzing include counting neighboring pixels of at least some pixels in the data array.
Another preference as to the above-mentioned selecting substep is that it include defining locations of the particular pixels to receive a particular colorant density for said particular colorant. In this case, an added preference is that the selecting substep further include generation of additional density levels for printing based upon density levels within the data array; and that this generation include defining locations of specific pixels among the particular pixels, to receive at least one other specified colorant density for the same particular colorant. Yet another preference, along this line, is that the method also include performing both applying steps with respect to at least one other particular colorant.
In preferred embodiments of a second of its aspects, the invention is a method of printing an image, by construction from individual colorant marks deposited in a pixel array on a printing medium, based on pixels of a corresponding original data array. The method includes the step of, based on the original array, defining an augmentation array.
For purposes of this document, the term xe2x80x9caugmentation arrayxe2x80x9d means an array of selected pixels of the image that is to receive an additional positive quantity of colorant, beyond the quantity specified by the original data array. As the additional quantity is explicitly positive, this aspect of the present invention differs from depletion.
This additional quantity does not extend to the trivial case of a redundant, or entirely repeated, second printing of the same colorant distribution provided by the original array. In other words, xe2x80x9caugmentationxe2x80x9d is to be understood as adding colorant in positions that are selected, rather than merely making an entire image darker (as for example in the case of overprinting an entire page of text, as a way of producing boldface type).
Also included is the step of applying the augmentation array to control a portion of deposition of colorant in forming said pixel array on the printing medium.
The foregoing may constitute a description or definition of the second facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention too significantly mitigates the difficulties left unresolved in the art.
In particular, by forming and using an augmentation array that is selectively based entirely upon the original data, it is possible to enhance colorant density with no need for additional input data. A small price is paid in data processing to select pixels for inclusion in the augmentation arrayxe2x80x94but as will be seen from later portions of this document this processing burden is very minor.
Although the second aspect of the invention thus importantly advances the art, nevertheless it is advantageously practiced in conjunction with certain characteristics or features that enhance enjoyment of its benefits.
For example, it is desirable to also apply the original array to control another portion of deposition of colorant in forming the pixel array on the printing medium. The augmentation-array applying step increases the deposition of colorant, relative to the original-array applying step, by less than a one hundred percent increase. It will be noted that these first two stated preferences in essence reinforce the underlying concept of an xe2x80x9caugmentation arrayxe2x80x9d as set forth above.
Another preference is that the applying step use the augmentation array to provide a nonlinear colorant-deposition response to the data array. Preferably this non-linear colorant-deposition response tends to compensate for a nonlinear responsexe2x80x94of the image-construction processxe2x80x94to the data array.
Another preference, particularly for use with a printing mechanism that passes a printhead plural times over a printing medium, is that the applying step include using the augmentation array to control operation of at least one pass of the printhead. Alternatively it is preferred, particularly for use with a printing mechanism that has plural printheads for a particular ink color and dilution, that the applying step use the augmentation array to control operation of at least one printhead.
Yet another alternative preferencexe2x80x94particularly for use with a printing mechanism that has plural sets of nozzles for a particular ink color and dilutionxe2x80x94is that the applying step use the augmentation array to control operation of at least one set of nozzles.
From the foregoing it will be understood that one way to implement this aspect of the invention is by directing to a printmasking stage (1) the original input data array, together with (2) the results of applying the augmentation array to the input data array. The printmasking function then distributes among plural printing passes all the pixels that make up the sum total of both data setsxe2x80x94analogously to the printmasking done in conventional systems such as plural-pass single-bit systems, C-Ret systems etc.
This second facet of the invention is not limited to colorant augmentation in separate printing passes, but rather is also applicable to any method of controlling a portion of colorant deposition in forming the pixel array. Also possible, for instance, is selective increase of dot size for the pixels selected by the augmentation array.
In preferred embodiments of a third basic aspect or facet, the invention is apparatus for printing an image on a printing medium, by construction from individual marks formed in a pixel array on a printing medium, based on pixels of a corresponding data array. The apparatus includes some means for analyzing pixels of the data array to identify areas of the data array that are locally dense.
For purposes of breadth and generality in discussing the invention, these means will be called simply xe2x80x9cthe analyzing meansxe2x80x9d. In addition the apparatus includes some means for applying the identified locally dense areas of the data array to enhance printing of the image on the printing medium by adding colorant. Again for generality and breadth these means will be called the xe2x80x9capplying meansxe2x80x9d.
The foregoing may represent a definition or description of the third major facet of the invention in its broadest or most general form. Even in this form, however, the contribution of this aspect of the invention to progress in the art can now be appreciated.
More specifically, this third aspect of the invention complements those discussed earlier. It selects pixels for use as the augmentation array of the second aspect, in performing the dual data application of the first aspect.
Nevertheless preferably this third facet of the invention is practiced in conjunction with certain additional features or characteristics that maximize enjoyment of its benefits. For example, preferably the analyzing means operate by counting inked data-array pixels adjacent to at least some of the data-array pixels, and the applying means apply additional colorant to identified locally dense areas.
Another preference is that the apparatus also include printhead devices for applying colorant, a carriage for moving those devices in a scan direction, printing-medium advance means providing relative motion between the carriage and medium at right angles to the scan direction, and an encoder system for developing carriage position and velocity signals; the analyzing means and applying means include a processor. In this case a further preference is that the analyzing means and the applying means each operate with respect to each available particular colorant.
This third facet of the invention, like the second, is applicable to any method of controlling a portion of colorant deposition in forming the pixel array. In other words this aspect of the invention is not limited to printing discrete colorant quanta in separate passes but ratherxe2x80x94as described above for the second facet of the inventionxe2x80x94can be implemented by dot-size increase if desired.